What i've been noticing is a lot of the same family of pet have similar strengths vs weaknesses so for example elementals do great against mechanical and if you had an entire team against a team with one elemental you would have no way of countering that pet

I think you can definitely do an all bot team if you wanted! I did an all critter team until level 15 and many of the pet battles were against beasts. It took longer to level, but it was totally worth it!

Unfortunately I don't have personal experience using those 3 at all and don't even have a Mini-Thor. The only thing I could see as a possible future problem is at level 10, Personal World Destroyer and Warbot both get an attack that increases frequency if it attacks first. According to my pet journal, both start with only 10 speed (vs 11 or 12.) I'm not 100% about their stat growth, but the level 10 attack may not be as beneficial as it could be if they had higher speed.

Also: Warbot may be classified as a "Common" pet because the item is "Common." I'm not 100% sure about this, though.

Ah, that could be a concern. To be honest, the bots' attacks kind of suck, at least compared to some I've seen. PWD's the only one with a heal, and it takes two turns to activate [although it does heal him to full].

lil XT if built around him might wreck house. Tyrant Tantrum is pretty good, and if you can get someone to trigger heartbreaker (Which is meh anyway), plus lightning storm, you might have a team killer on your hands.

If you don't fight any elementals, you have a good chance to win! Be careful of Lighting Storm though, as it causes ALL PETS to do bonus mechanical damage. I took a beating when I used this and then the opponent used an attack that hit multiple times - each hit did +10 mechanical damage for an extra 30 damage to me ;(

I think the above recommendation of a lifelike toad seems good vs elementals, as it uses all aquatic attacks. I may try it :P